1. Field of the Invention
The present invention relates to apparatus for aligning mask and wafer used in manufacturing semiconductor circuit elements in a predetermined positional relation.
2. Description of the Prior Art
In manufacturing IC, LSI, VLSI and the like, there is employed usually a method in which a plural number of masks each having circuit patterns thereon are used so as to print the patterns successively on a wafer. To carry out this known method it is absolutely necessary to bring the circuit pattern already printed on the wafer and a circuit pattern next to be printed on it into a predetermined, aligned relation. For this purpose, an alignment apparatus is used which moves one of the mask and wafer relative to the other to establish a desired alignment between the circuit pattern on the mask and that on the wafer.
In the alignment apparatus conventionally used hitherto, the wafer is supported movably relative to the mask and the operator adjusts the relative position between the wafer and the mask while observing the pattern on the former and that on the latter through a microscope.
Also, such type of automatic alignment apparatus has been known in which use is made of mask and wafer each having alignment marks provided thereon and these alignment marks are scanned to photoelectrically read out the alignment marks. During scanning there is detected the amount of deviation between the mark on the mask and that on the wafer, by means of which the wafer is moved in x,y and .theta. directions for alignment.
When alignment marks are provided in an area between circuit patterns, a more effective use of surface area is allowed for both mask and wafer. Such improved type of mask and wafer are disclosed in U.S. Pat. No. 3,796,497.
In a preferred embodiment of the invention, alignment marks are provided in a narrow strip like area of 100 .mu.m on mask and wafer and the diameter of beam spot used in scanning the alignment mark is in a order of 10 .mu.m. In this case, the scanning line, that is, the position on which the beam spot impinges should be exactly coincident with the narrow strip area. It is a common practice in the art to feed wafers to the alignment apparatus after carrying out a pre-alignment for them by an automatic feeding apparatus. However, the automatic feeding apparatus commonly used for this purpose is not sufficiently accurate to effect a coincidence of the narrow strip area and scanning line as mentioned above. So long as the coincidence is not established, no alignment operation is possible.